The present invention applies to a hydraulic pressure transmitter for at least one receiver mechanism, such as, a brake cylinder. The pressure transmitter can be of the type which consists of a small cross-section piston directly activated by a push rod to pressurize and reduce the volume of a small cross-section hydraulic chamber linked to the receiver mechanism; and a large cross-section piston activated parallel to the push rod by or via an elastic mechanism of transmission, such as, a calibrated spring, to pressurize and reduce the volume of a large cross-section hydraulic chamber also linked to the receiver mechanism. Such a hydraulic pressure transmitter, in order to function at the beginning of transmitter activation, delivers a large flow of hydraulic fluid by the large piston until the delivery pressure, acting on the large diameter piston, is able to oppose the action of the elastic mechanism of transmission.
Furthermore, such piston-type hydraulic pressure transmitters--commonly called "master cylinders" in the automotive industry--were proposed to allow the wear of the friction mechanism of the brakes to be automatically compensated so that the friction linings be rapidly brought into contact with their braking element through a large flow of low pressure braking fluid, and that the application of the brakes then continue at elevated hydraulic pressure after initial application of the friction linings. This method of brake control made it possible to insure application of the brakes in a braking system with relatively significant play between the friction lining and braking element.
These hydraulic pressure transmitters with double-braking pressure (initial application pressure and continued application pressure) were no longer used when reliable and efficient slack-adjusting regulators were available, and the use of disc brakes in the automotive industry made it possible to implement follower hydraulic systems which limited the application stroke of the friction linings. Typically, disc brake units use an elastic follower segment which prevents, by friction, any considerable reversing of the friction linings after applying the brakes; but allows, on the other hand, the forced reverse of these linings to proceed to their changing.
For certain hydraulic pressure-controlled, high-power disc brakes, as used, for instance, on heavy road vehicles or railroad vehicles (particularly those equipped with floating calibers), it is necessary to obtain rapid and complete disengagement of the brakes to authorize a reverse of the friction linings at disengagement such that considerable play (on the order of a millimeter, at least, for high-power disc brakes) is left between the friction linings and the braking elements. These brakes, which have high-temperature braking power, also require, upon application of the brakes, considerable elastic work which corresponds to the compression of the relatively thick friction linings and to the elastic extension of the calibers which hold the friction linings. These various requirements for activation of high-temperature power brakes, particularly the disc brakes of heavy road and railway vehicles, cannot be completely satisfied by the known systems of double-piston master cylinders. Actually, in these known systems, the leak-proof linings for large cross-section pistons cannot withstand the high pressures required for activation of the brakes. Furthermore, in the systems where the low pressure chamber is isolated from the high pressure chamber, the low pressures obtained by oil insufflation or by piston scraping are insufficient to insure application of the friction linings against power return springs used to achieve an inactive position.
It is a requirement, therefore, that such a hydraulic pressure transmitter transmit an initial large flow of hydraulic fluid at already significant pressure (between 2 and 10 bar), sufficient to overcome the loss of head of the hydraulic circuit to ensure rapid application of the friction linings to the braking discs by acting against these powerful return springs, which tend to return these linings to a disengaged position. The return springs must act under all friction conditions in order to guarantee rapid disengagement of the brakes. They are particularly indispensable to prevent locking of the braked wheels, i. e., blocking of these wheels or tendency to block during braking and before the stop of the braked vehicle.